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BASIC STUDY ON THE INFLUENCE OF GLASSCOMPOSITION AND AL CONTENT ON THE Ag/Al PASTECONTACT FORMATION TO BORON EMITTERS

S. Körnera, F. Kieferb, R. Peibstb, F. Heinemeyerb, J. Krügenerc, M. Ebersteina

a) Fraunhofer IKTS, Winterbergstr. 28, 01277 Dresden, Germany; b) Institut für Solarenergieforschung GmbH Hameln/Emmerthal Am Ohrberg 1, 31860 Emmerthal, Germany; c) Institute of Electronic Materials and Devices, Leibniz Universität Hannover, 30167 Hannover, Germany

© Fraunhofer

2/22

Introduction & motivation

monocrystalline n-type Si will gain market share up to 39%[1]

Advantages of n-type solar cells[2-4]:

Absence of light induced degradation

Higher bulk life times

Insensitivity to common metal impurities

Disadvantage of n-type solar cells[5]:

Depleted emitter zone at the wafer surface

Solution:

Controlled etch back of depleted zone risk of damaging

Al addition Al-Si-alloy formation for lowering contact resistance

Basic study on influence of Al on paste sintering behavior

[1] www.itrpv.net; [2] www.lg-solar.com; [3] D. Song, et. al., Proc. of the 38th IEEE-PVSC 2012 Austin, TX, USA, pp. 003004-003008; [4] V. D. Mihailetchi, et al., Proc. of the 25th EU-PVSEC 2010, Valencia, Spain, pp. 1446-1448, [5] R. Lago et al., Prog. Photovolt: Res. Appl. 2010; 18:20–27, DOI: 10.1002/pip.933.

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3/22

CHIP: cell concept

Cell architecture: PERT (passivated emitter passivated rear totally doped)

n-Typ Si

base

n+-Si

p+-Si

SiN

metal

Al2O3

Components [Vol%]

paste Glass silver aluminium

PbO containing PbO free

1 5 95 0

2 5 92,5 2,5

3 5 87,5 7,5

4 5 95 0

5 5 92,5 2,5

6 5 87,5 7,5

Ag

Al

PbO-glass

Ag

Al

PbO free-glass

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4/22

Cross sections in dependence of glass frit and Al content

PbO glass PbO-free glass

0 Vol% Al

2.5 Vol% Al

7.5 Vol% Al

Tf = 810°C

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5/22

Single components: glasses

(„long“ h-T-chart) („short“ h-T-chart)

electrical conductivemelt

PbO PbO free

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6/22

Cross sections in dependence of glass frit and Al content

PbO glass PbO-free glass

0 Vol% Al

2.5 Vol% Al

7.5 Vol% Al

Tf = 810°C

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7/22

Cross sections in dependence of peak temperature andAl content – lead oxide glass frit

With increasing peak firing temperature increasing glass layer thickness[6,7]

At higher temperatures precipitates grown in silicon surface

[6] M. Eberstein et al. 2012, Energy Procedia, Vol. 27, pp. 522– 530. [7] Z. G. Li et al., J. Appl. Phys. 110, 074304 (2011); doi: 10.1063/1.3642956.

780°C, 0 Vol% Al 810°C, 0 Vol% Al 840°C, 0 Vol% Al

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8/22

Cross sections in dependence of peak temperature and Al content – lead oxide glass frit

With addition of 2.5% Al inhomogeneous glass layer

Enrichment of glass in vicinity of former Al particle

Depletion of glass at the interface, Ag-Al-spikes at the wafer surface

780°C, 0 Vol% Al 810°C, 0 Vol% Al 840°C, 0 Vol% Al

780°C, 2.5 Vol% Al 810°C, 2.5 Vol% Al 840°C, 2.5 Vol% Al

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9/22

EDX analysis of cross section

2.5 Vol% Al content, PbO glass, fired at 780 °C

Al-Ag-alloy indicated by Ag located together with Al

Pb and O indicating the glass can be found in the vicinity of the former Al grain

Si Al Pb O Ag C

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10/22

Cross sections in dependence of peak temperature andAl content – lead oxide free glass frit

With increasing peak firing temperature increasing glass layer thickness[6,7]

At higher temperatures silver colloids precipitates in glassy interface layer

780°C, 0 Vol% Al 810°C, 0 Vol% Al 840°C, 0 Vol% Al

[6] M. Eberstein et al. 2012, Energy Procedia, Vol. 27, pp. 522– 530. [7] Z. G. Li et al., J. Appl. Phys. 110, 074304 (2011); doi: 10.1063/1.3642956.

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11/22

Cross sections in dependence of peak temperature andAl content – lead oxide free glass frit

Same effects as mentioned for PbO-glass

But higher melting glass less glass at the interface

Small spikes of Ag-Al-alloy at higher temperature

780°C, 2.5 Vol% Al 810°C, 2.5 Vol% Al 840°C, 2.5 Vol% Al

780°C, 0 Vol% Al 810°C, 0 Vol% Al 840°C, 0 Vol% Al

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12/22

In-situ-contact resistance measurement assembly

quasi 4-point measurement on silver contact layers

I

I

U

PV belt furnace

Data logger

Measurement of the contact formation during firing the solar cells

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13/22

In-situ-contact resistance – lead-oxide glass frit

0 20 40 60 80 100 120

0,1

1

10

100

1000

0 20 40 60 80 100 120

0

100

200

300

400

500

600

700

800

900

resis

tance [O

hm

]

time [s]

R

Tem

per

ature

[°C

]time [s]

T-Profil

Lead-oxide paste with 0 Vol% Al

Peak firing temperature 840°C (@IKTS)

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14/22

In-situ-contact resistance – lead-oxide glass frit

0 20 40 60 80 100 120

0,1

1

10

100

1000

0 20 40 60 80 100 120

0

100

200

300

400

500

600

700

800

900

0 20 40 60 80 100 120

0

2

4

6

8

10

12

resis

tance [O

hm

]

time [s]

R

Tem

per

ature

[°C

]time [s]

T-Profil

gla

ss v

iscosity [P

a*s

]

time [s]

glass A

Lead-oxide paste with 0 Vol% Al

Peak firing temperature 840°C (@IKTS)

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15/22

In-situ-contact resistance – lead-oxide glass frit

[7] M. Eberstein et al., In-situ Observations of Glass Frit Related Effects during the Front Side Paste Contact Formation, 40th PVSC 2014, June 8-13, 2014, Denver, CO.

Lead-oxide paste with 0 Vol% Al

Peak firing temperature 840°C (@IKTS)

glass viscosity dependent ARC etching

Intensity of interface reconstruction

0 20 40 60 80 100 120

0,1

1

10

100

1000

0 20 40 60 80 100 120

0

100

200

300

400

500

600

700

800

900

0 20 40 60 80 100 120

0

2

4

6

8

10

12

resis

tance [O

hm

]

time [s]

R

Tem

per

ature

[°C

]time [s]

T-Profil

104.4

Pa*s

gla

ss v

iscosity [P

a*s

]

time [s]

glass A

658°C 485°C

© Fraunhofer

16/22

In-situ-contact resistance – lead-oxide glass frit

0 20 40 60 80 100 120

0,1

1

10

100

1000

0 20 40 60 80 100 120

0

200

400

600

800

0 20 40 60 80 100 120

0

2

4

6

8

10

12

resis

tance [O

hm

]

time [s]

R

Tem

per

ature

[°C

]

time [s]

T-Profil

655°C 645°C

gla

ss v

iscosity [P

a*s

]

time [s]

glass A

104.4

Pa*s

Lead-oxide paste with 0 Vol% Al (left) and 7.5 Vol% Al (right)

Peak firing temperature 840°C (@IKTS)

Al decreased intensity of contact formation reaction

0 20 40 60 80 100 120

0,1

1

10

100

1000

0 20 40 60 80 100 120

0

100

200

300

400

500

600

700

800

900

0 20 40 60 80 100 120

0

2

4

6

8

10

12

resis

tance [O

hm

]

time [s]

R

Tem

per

ature

[°C

]time [s]

T-Profil

104.4

Pa*s

gla

ss v

iscosity [P

a*s

]

time [s]

glass A

658°C 485°C

© Fraunhofer

17/22

In-situ-contact resistance – lead-oxide glass frit

0 20 40 60 80 100 120

0,1

1

10

100

1000

0 20 40 60 80 100 120

0

100

200

300

400

500

600

700

800

900

0 20 40 60 80 100 120

0

2

4

6

8

10

12

resis

tance [O

hm

]

time [s]

R

Tem

per

ature

[°C

]time [s]

T-Profil

104,4

Pa*s

gla

ss v

iscosity [P

a*s

]

time [s]

glass A

658°C 485°C

0 20 40 60 80 100 120

0,1

1

10

100

1000

0 20 40 60 80 100 120

0

200

400

600

800

0 20 40 60 80 100 120

0

2

4

6

8

10

12

resis

tance [O

hm

]

time [s]

R

Tem

per

ature

[°C

]

time [s]

T-Profil

655°C 645°C

gla

ss v

iscosity [P

a*s

]

time [s]

glass A

104.4

Pa*s

Reduction of Ag2O with Al is energetic convenient than reduction with Si

© Fraunhofer

18/22

In-situ-contact resistance – lead-oxide free glass frit

Lead-oxide free paste with 0 Vol% Al (left) and 7.5 Vol% Al (right)

Peak firing temperature 840°C (@IKTS)

contact formation is improved through Al-addition

0 20 40 60 80 100 120

0

200

400

600

800

0 20 40 60 80 100 120

0,1

1

10

100

1000

Tem

per

ature

[°C

]time [s]

T-Profil

609°C

resis

tance [O

hm

]

time [s]

R690°C

0

2

4

6

8

10

12

glass B

gla

ss v

iscosity [P

a*s

]

105.0

Pa*s

0 20 40 60 80 100 120

0,1

1

10

100

1000

0 20 40 60 80 100 120

0

200

400

600

800

0 20 40 60 80 100 120

0

2

4

6

8

10

12

resis

tance [O

hm

]

time [s]

R

Tem

per

ature

[°C

]

time [s]

T-Profil

694°C 624°C

105,0

Pa*s

gla

ss v

iscosity [P

a*s

]

time [s]

glass B

© Fraunhofer

19/22

0% Al 2.5% Al 7.5% Al 0% Al 2.5% Al 7.5% Al

-1,0

-0,8

-0,6

-0,4

-0,2

0,0

rela

tive

effic

ien

cy

aluminium content

780°C

810°C

840°C

Electrical characterization of prepared cells

Paste without Al shows improving electrical performance with increasing firing temperature

Addition of aluminum decreases efficiencies opposite trend in terms of firing temperature

PbO-free glass

PbO glass

© Fraunhofer

20/22

0% Al 2.5% Al 7.5% Al 0% Al 2.5% Al 7.5% Al

-1,0

-0,8

-0,6

-0,4

-0,2

0,0

rela

tive

effic

ien

cy

aluminium content

780°C

810°C

840°C

Electrical characterization of prepared cells

PbO-free glass

PbO-free paste without Al shows improved electrical performance with increasing firing temperature

With the addition of Al a gain of efficiency can be reached

PbO glass

© Fraunhofer

21/22

Conclusions

Basic study of Contact formation of silver metallization paste on n-type Si in terms of Al content

Addition of Al

lowers densification

leads to modified glass distribution in the interface structure glass is enriched in the vicinity of Al

Reduction of Ag2O solved in the glass is most likely with Al than with Si

In-situ resistance measurement:

ARC etching is glass viscosity depended

Intensity of interface reconstruction can be controlled with Al addition

© Fraunhofer

22/22

www.ikts.fraunhofer.de

The authors thank all involved scientists and technical staff…

… and you for your kind

attention!

© Fraunhofer

23/22

Experimental setup

Pastes were varied according to

Aluminum content

Glass frit chemistry and viscosity

Cleaning/damage etch

Single side texture

Boron implant (FS)

Phosphorous implant (RS)

Co-annealing

RS passivation (SiNx)

FS passivation (Al2O3+SiNx)

FS screen print (9 wafer per paste)

Drying/firing (3 wafer per T)

RS Laser contact openings

Al evaporation

Contact tempering